In order to decide among several concurrent access requests from different components, arbitration circuitry is needed which must determine the relative ranks of the requesting components and enable the highest-ranking one to seize the data bus for one or more operating cycles. In a complex system having a large number of such components served by the same bus, the decision-making procedure is advantageously distributed among a multiplicity of arbitration devices which are individually associated with these components and can operate rapidly on the basis of certain criteria. Not infrequently, access requests must be responded to in a time interval of 125 ns or less in keeping with corresponding operating cycles of the data bus. Such a distribution scheme is also advantageous, as compared with a centralized structure, in that it facilitates reconfiguration in the event of failure of one or more components.
In many instances, moreover, the relative rankings of the components involved are not fixed but can be varied in order to improve the access possibilities of components which otherwise would be at the bottom of a rigid order of precedence. Such modifications, again, should be carried out rapidly during a fraction of an operating cycle of the associated data bus.